Why are people in Nineteenth Century photos usually grimacing? This newlywed couple looks like they've just been sentenced to hard labor. American author Mark Twain explains their expression:

A photograph is a most important document, and there is nothing more damning to go down to posterity than a silly, foolish smile caught and fixed forever.

Twain wasn't alone. He supported a traditional though fading belief that smiling made you look stupid. The Atlantic cites scholar Nicholas Jeeves:

Twain wasn’t the only believer in the idiocy of the style. Look back at painted portraiture — the tradition photography inherited — and you’ll rarely see a grinning subject. This is, in fact, Jeeves’s subject. “By the 17th century in Europe,“ he writes, “it was a well-established fact that the only people who smiled broadly, in life and in art, were the poor, the lewd, the drunk, the innocent, and the entertainment.”

Indeed, not only were smiles of the middling sort, they breached propriety. In 1703, one French writer lamented “people who raise their upper lip so high… that their teeth are almost entirely visible.” Not only was this discourteous, he asked: Why do it at all? After all, “nature gave us lips to conceal them.”

Portraits represented an ideal. It’s easy to mock them — they were the profile pictures of the aristocracy, in a sort of way — but they were crucial, tied to mortality, a method of preserving a person’s visage and affect. Jeeves puts it well: “The ambition [with portraiture] was not to capture a moment, but a moral certainty.” Subjects never looked exactly like their picture, yet their portraits were how they appeared. Portraits had permanence. You did not want to commit a permanent faux pas.

Quoting a 17th century European source about 19th century American photographs seems rather silly and pointless. In the course of my historical research, I read entire runs of newspapers from the 19th century on Google Newspaper archive and this issue did, in fact, come up in an opinion piece (sadly, I don't recall the paper but it was probably one from Nova Scotia.) The writer opined, much as Twain did, that it was risky to have to hold a smile until it looked forced, fixed, and frozen (and stupid), so, even though it made people look grim, they usually chose to simply let the face relax as naturally as possible so the position could be held. Photographs were expensive and people were loath to take a risk of spending the money on something that would look foolish.